Microtine rodents undergo regular 3-4 year cycles of density; however, there is no general agreement as to what mechanisms control this phenomenon. One of the prime diffculties in sorting out the various approaches is the lack of a functioning design for experimental manipulation. Most decisive experiments cannot be done now because of the general failure to field experiments due to problems of dispersal. I suggest in this proposal to test an operational method of performing field experiments. This method makes use of the natural process of dispersal in field populations. Lidicker (1975) proposed the term "Dispersal Sink" to refer to: "Space which provides an outlet for wandering impulses...". I propose to build fenced-in enclosures that contain sub-optimal habit which will be continuously trapped out in order to maintain the dispersal sink. Two variations will also be tested: an open-ended enclosure emptying into sub-optimal habitat and a grassland enclosure with one end maintained as a dispersal sink by ccntinuous trapping out of mice. If the methodology is successful, decisive experiments can be performed in future years to sort out the proposed mechanisms of population cycles.